I'm reminded of when we first learned of the game and you made a joke about how much lore surrounds the first boss of FF1 who was just some rogue knight originally lol. You were so right. If you don't mind spoilers, I'm just gonna summarize the plot here:
Jack Garland is a "Stranger", which is basically an operative from some extra-dimensional society called the Lufenians. He and his friends Jed and Ash are sent to Cornelia to be "Warriors of Light" whose job is to maintain the balance of light and dark in the world. The Lufrenians are basically trying to terraform the world of FF1 into the place where their perfect society would be, and this involves importing locations into it from other dimensions (each dimension is listed by number and obviously corresponds to the same numbered FF game) The memories of the "Strangers" are wiped before going there, and they've been sent countless times as the Lufenians try to perfect the balance. The Lufenians have the ability to reset the world of Cornelia as they see fit, so the Strangers relive the same events again and again. There were two other characters, Neon and Sophia, who were previous Warriors of light who died on previous cycles and became Fake!Garland and Tiamat respectively after they died, but are revived after Jack defeats them, and join the group. Astos, the King of Dark Elves... this guy:
is a biologically created servant of the Lufenians who is supposed to help Jack and co. on each loop. This takes a while to come together, but it is revealed that Jack on a previous cycle learned the truth of their situation and started conspiring with Astos to overthrow the Lufenians and break the cycle. The Lufenians are like Spock, all logic and order, so Astos and Jack make a plan for Jack to become Chaos in another Cycle. Jack promises to remember this plan, but he doesn't on future cycles, which causes Astos to become bitter and hateful toward him, (they were
gay lovers BFFs in that one loop because Jack recognized Astos' worth as an individual and risked his life for him, so it hurts Astos that Jack doesn't remember) even though he can't blame Jack for forgetting, but ultimately the hate inside him causes him to become a monster which the party has to kill. Only after his death does everyone regain their memories of the plan (the ORBS, er, crystals they brought with were supposed to keep them from remembering, but once they got filled up with darkness Astos was supposed to extract them and send them back to the Lufenians, but since he was conspiring with Jack this time he didn't do that and everyone kept gaining more memories as they killed more monsters). All the Darkness that's released after Astos dies causes monsters to invade Cornelia, and all the citizens die, including princess Sarah (was was kind of a love interest to Jack), and in order to push Jack over the emotional tipping point so he becomes Chaos, the rest of the party starts trying to kill him.He ends up killing them, and his grief causes him to go crazy and gain Chaos powers. The extraction point to return to Lufenia was in the Chaos Shrine, so Jack returns there with the plan to steal their ability to reset time and kill them all. They try to reset time while he's there to stop him, but he uses his darkness powers to break and steal the "crystal matrix" that lets them do it, and all the darkness that's released forms into FF1 Chaos... this guy:
Who Jack fights and steals the power of. The Lufenians, in a last ditch effort to rid themselves of Jack, eject the entire pocket dimension he's in and send him back to Cornelia. He leaves them with the cryptic message that they don't need to send any more strangers, which he doesn't explain the meaning of to them. We see him sitting on the throne in the Chaos Shrine, all his dead friends back with him as the four fiends (the ones you killed in the main game are implied to be versions of them from past cycles who were corrupted by the bitterness and hate they felt in death)... these guys:
and it seems Jack's new plan to get back at the Lufrenians is to create new Warriors of Light born and raised in Corneria itself rather than some future pocket dimension society, who will be true warriors of light and be able to ACTUALLY save the world instead of just making things worse like his group did. The final scene is Garland sitting on his throne, with the four warriors of light from FF1 (using the Dissidia design for the main one) coming in to face him. And so the grand origin was told... of this guy:
And before you think all the extra-dimensional society stuff was new, apparently that also ties into FF1 because the Lufenians are IN that game as the survivors of an advanced civilization that was decimated by Tiamat:
https://finalfantasy.fandom.com/wiki/Lufenia_(Final_Fantasy)
So yeah, Jack got his revenge in the end lol.
Mainly because I have been doing co-op with my streamer friend and another of his viewers we have just been zipping through these dungeons ... it has been hilarious, and chaotic. Might start being even more selective when playing myself though examining green and purple gears when I pick them up then dropping it back down if I don't need or want it. Letting go of the I have to have at least five of everything from everywhere for everyone for fashion mentality will probably help as well 
